The accord also provides for the clearance of heavy weapons from a U-shaped zone around the northwestern region of Idlib, but a monitor said rebels late Saturday fired mortar shells from it.

The deal agreed last month between regime ally Russia and rebel backer Turkey is only the latest in a string of truces throughout Syria’s seven-year war, which has killed more than 360,000 and displaced millions.

It calls for setting up horseshoe-shaped buffer zone around the Idlib region that would be free of heavy arms by October 10 and of “radical fighters” by Sunday at midnight. But with just hours to go on Sunday, the Syrian Observatory for Human Rights monitor and an AFP correspondent in Idlib said there was still no sign that they had left so far. And deadly mortar rounds fired late Saturday from the planned buffer zone, according to the Britain-based monitor, appeared to indicate the first part of the deal was not fully implemented.

“In a few hours, the buffer zone is supposed to enter into force, but the deal has not yet been implemented,” said Rami Abdel Rahman, the head of the Britain-based Observatory.

Late Saturday, “heavy mortar shells” were fired from the planned buffer area into regime territory, killing two soldiers, the Observatory said.

Rebels and jihadists had reportedly fulfilled the first part of the deal, with Turkish officials, armed factions and the Britain-based Observatory reporting that the area was free of heavy-duty weaponry.

But the shells which Saturday hit an army position in Hama province appear to have violated the accord.

“This is the first clear violation of the deal since the heavy weapons were withdrawn. This area is supposed to be clear of heavy weapons, including mortar shells,” Abdel Rahman said.

He said intermittent regime shelling had been hitting the planned buffer for days, but the deal does not require government forces to withdraw any of their weapons. On Sunday evening, regime shelling hit an area in the west of the planned buffer zone, in the northwest of Hama province, Abdel Rahman said.

Syrian pro-regime daily Al-Watan also reported rebel shelling, saying on Sunday that western parts of Aleppo province were being hit with “rocket fire and shelling with heavy weapons, which were supposed to be pulled out from the area”.

And an AFP correspondent in western Aleppo reported mortar fire in the area after several days of quiet. The Observatory said it was unclear which groups fired the mortars late Saturday, as both the Turkish-backed National Liberation Front and rival jihadist factions were present in the area.

The NLF - which holds just under half of the Idlib region and has welcomed the accord - did not immediately respond to AFP’s request for comment on the shelling. The lion’s share of Idlib is held by Hayat Tahrir al-Sham (HTS), an alliance led by former Al-Qaeda members, as well as more hardline jihadists like Hurras al-Deen and Ansar al-Islam.Those fighters also control more than two-thirds of the planned buffer zone and are supposed to withdraw by Monday.

“If HTS acts as a spoiler to the agreement on the ground, this will probably lead to one of two scenarios: either Turkey and the NLF launch military action against HTS, or Russia will seize the opportunity with the support of the regime and its allies to enter Idlib,” he said.

“The ramifications of that move could be vast,” he added.

Syrian President Bashar al-Assad and other top government officials have awarned that the Idlib deal was only a “temporary” measure.

On Friday, residents around Idlib received warning messages on their mobile phones from the Syrian army.